The present invention relates to a multilayer tube with ends suited to butt welding.
In particular the present invention relates to tubes realized with an internal layer of material suited to preservation of the external layer from the aggressive action of the fluid conveyed but with only the external layer of material enjoying the weldability features desired for the tube union.
In the field of tubing the advantages of tubing of polyethylene (PE) and similar materials are well known. For example, flexibility, lightness, environmental corrosion resistance and the possibility of using a choice of one of three different welding technologies (socket, butt and induction melting) allow resolving effectively any application problem in the conveyance of fluids such as water and gas. Polyethylene however is not resistant to hydrocarbons and in particular to solvents. This has in fact limited its use in industry and in particular oil applications.
To obviate the limitations of PE there was recently developed a new technology going under the name of xe2x80x98multilayer tubingxe2x80x99. The technique consists of co-extruding inside the polyethylene of a thin layer (termed xe2x80x98linerxe2x80x99) of thermoplastic resistant to hydrocarbons, for example nylon, EVOH, PVDF et cetera. The internal layer prevents the hydrocarbons from coming into contact with the polyethylene which, thanks to its good mechanical features and low cost, constitutes the structural layer of the tubing and obviates the limitations of tubing of only PE.
Since the materials used as the liner are not compatible with polyethylene, to obtain perfect adhesion between the layers one or more adhesives termed xe2x80x98tie-layerxe2x80x99 are used. Although this technique offers a solution to the realization of relatively economical tubes of low thickness and resistant to hydrocarbons it introduces the problem of easy and convenient joining of tubes.
To seek to remedy this new problem techniques have been proposed using special unions or sleeves with chambers receiving the tube head and welded by thermoelectric adhesion systems on the outer wall of the PE tube. For relatively large diameters, for over 160 mm, the electrical sleeves are however rather costly.
In the prior art there are no reliable solutions for more economical and easier butt welding.
The general purpose of the present invention is to remedy the above mentioned shortcomings by making available a butt welding method and a multilayer tube allowing simple and fast butt welding.
In view of this purpose it was sought to provide in accordance with the present invention a weldable plastic tube having at least one end designed to be butt welded and designed for the conveyance of a fluid producing a drop in mechanical resistance of said weldable plastic and comprising an internal layer for protection of the plastic of the tube from contact with the fluid conveyed and with a terminal section of the tube adjoining said welding end permanently covered with an external reinforcing sleeve also of weldable plastic to participate in the butt welding of the tube with the sleeve having a thickness such as to make the mechanical resistance of the tube section nearest the welded end when in contact with the fluid in the welding zone without the internal protective layer at least equal to the mechanical resistance of the tube not in contact with the fluid.
A butt welding method is also claimed.